In the case where the semiconductor switching element is connected to an inductive load and turn-on and -off of electric power supply to the inductive load is controlled by the semiconductor switching element, it is necessary to consume energy accumulated in the inductive load within the circuit at the time of turn-off. Energy E accumulated at this time becomes E=½×LI2 with self-inductance and a current being designated as L and I.
In the case where the semiconductor switching element is a MOSFET consisting of silicon, it takes a device structure having a spurious diode of an anti-parallel connection where its cathode is connected to its drain and its anode is connected to its source between the drain and the source. Therefore, since energy from the inductive load can be consumed employing an avalanche area of a parasitic diode when the MOSFET is turned OFF, the MOSFET semiconductor switching element has comparatively large avalanche maximum energy.
Incidentally, the avalanche maximum energy is an index of a breakdown endurance amount of the device, and is defined as a maximum energy that the device can consume without resulting in breakdown when the device consumes the energy accumulated in the inductive load.
In contrast, in the case where GaN-HEMT or GaAs-HEMT made of compound semiconductors is used as the semiconductor switching element, usually, the energy from the inductive load cannot be consumed inside the element, and exceeds the avalanche maximum energy between the gate and the drain (BVgd) and a source-drain breakdown voltage (Bvdsoff), which brings about element breakdown. Therefore, in a system of an inductive load with self-inductance L, such as an inverter, it is ordinary that the semiconductor switching element is used together with the protecting element.
For example, there is a method whereby an external diode is anti-parallel connected between the source and the drain of the HEMT as the protecting element. This method realizes the same structure as that of the MOSFET of silicon by including an external diode, and the structure consumes the energy from the inductive load. However, since energy when the HEMT is turned OFF from a state where a rated current is flowed to the HEMT will be consumed by the diode side, a large current comparable with that of the HEMT is required to flow in the diode, which poses a difficulty that the diode becomes large in size.
Therefore, Patent Literature 1 proposes a structure in which Zener diodes are anti-parallel connected between the gate and the drain and between the source and the gate as the protecting element. By this method, when a drain voltage increases by energy of the inductive load, at the same time of breakdown of the Zener diode between the gate and the drain, the Zener diode between the source and the gate also breaks down and a breakdown current flows. A voltage that is divided according to the number of stages of the Zener diodes is applied to the HEMT as a gate voltage. This breakdown current charges the gate like the case where the HEMT normally turns ON, which opens the channel and makes the HEMT turn ON. That is, the energy of the inductive load is consumed by the HEMT by making the HEMT turn ON. Therefore, the Zener diode only needs to be configured so as to flow a small current that can drive the HEMT, and therefore the size of the diode can be made small.